Jackson et al.—Behaviour of a social ant-like spider New Zealand Journal of Zoology, 2008, Vol. 35: 225–235 0301–4223/08/3503–0225 © The Royal Society of New Zealand 2008
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The natural history of Myrmarachne melanotarsa, a social ant-mimicking jumping spider Robert R. Jackson1,2 Ximena J. Nelson1,3,* Kathryn Salm1 1 School of Biological Sciences University of Canterbury Private Bag 4800 Christchurch 8140, New Zealand 2 International Centre of Insect Physiology and Ecology (ICIPE) Thomas Odhiambo Campus PO Box 30 Mbita Point, Kenya 3 Centre for the Integrative Study of Animal Behaviour Macquarie University Sydney, NSW 2109, Australia * Author for correspondence:
[email protected]. Abstract Myrmarachne melanotarsa, an ant-like jumping spider (Salticidae) from East Africa, is an accurate mimic of Crematogaster sp. and associates unusually closely with its models. M. melanotarsa is remarkable in that it forms dense aggregations and builds large nest complexes (numerous individuallyoccupied nests connected to each other by silk). Other salticids (Pseudicius spp., Menemerus spp.) live with M. melanotarsa in the same nest complex. These aggregations, which can exceed 50 conspecific individuals per colony, are considerably larger than those few previously described, and seem to have primarily a protective function. We provide baseline information on the natural history of M. melanotarsa, paying particular attention to predatory behaviour and association with Crematogaster sp., and fit this within current theory on the function of sociality in spiders. Other unusual behaviour of
Z07052; Online publication date 1 July 2008 Received 23 October 2007; accepted 8 March 2008
M. melanotarsa includes “mouthing”, in which the spider opens and closes its chelicerae while pressing its mouthparts against nest silk. We investigated the role of prior presence of Crematogaster sp. on nest silk in eliciting this previously unreported behaviour. Keywords Batesian mimicry; Crematogaster; Myrmarachne; Salticidae; social spiders INTRODUCTION Sociality in spiders is of exceptional interest, because the popular impression is that these predatory arthropods are highly aggressive toward members of their own species and prone to cannibalism, behaviour that would not appear conducive to sociality. Among more than 5000 species in the largest spider family, the Salticidae or jumping spiders, examples of sociality are especially scarce. The better-known examples of sociality come instead from web building species belonging to other families (e.g., Avilés 1997; Avilés et al. 2001; Whitehouse & Lubin 2005). Although terms such as “eusocial” and “semisocial” have been given strict definitions in the insect literature, we prefer to use the broader, everyday terms “social” and “sociality” without formal definitions, because they are routinely and casually used for species that tend to form groups of conspecific individuals (see Wilson 1975). Having unique, complex eyes that support exceptional spatial acuity (Land 1985; Land & Nilsson 2002; Harland & Jackson 2004), most salticids are solitary hunters that spend their lives outside webs (Richman & Jackson 1992). Yet there are examples of salticids aggregating into apparently sociable groups. Numerous salticid species from temperate regions occasionally form overwintering aggregations of clustered individual nests under the bark of trees and under stones, which are abandoned at the end of winter (Kaston 1948; Crane 1949; Jennings 1972). With the exception of cohabitation (when an adult male spins a nest alongside a nest of a subadult
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Fig. 1 Large colony of Myrmarachne melanotarsa. Nest complex in hole in tree caused by limb loss.
(one instar short of maturity) female and mates with her when she matures), which is almost universal in the Salticidae (Jackson 1986a; Jackson & Pollard 1997), the function of aggregating in salticids is poorly understood.
New Zealand Journal of Zoology, 2008, Vol. 35 The largest aggregations of salticids are formed by about a half dozen species that live together in nests interconnected by silk (“nest complexes”, Fig. 1, 2) in habitats near the shore of Lake Victoria in Kenya and Uganda (Jackson 1986b,c; Wesolowska & Salm 2002; Wesolowska 2006). More than one salticid species sometimes share the same nest complex. One of the species found in these nest complexes, Myrmarachne melanotarsa, is easily mistaken for a social insect, as all species in the genus Myrmarachne (Wanless 1978) are ant mimics. Batesian mimics are palatable individuals that deceive potential predators by resembling unpalatable models to which the predators have an aversion (Wickler 1968; Edmunds 1974, 1978; Vane-Wright 1980; Ruxton et al. 2004) and there is now considerable evidence that Myrmarachne is a genus of Batesian ant mimics (Cutler 1991; Cushing 1997; Edmunds 2006; Nelson & Jackson 2006; Nelson et al. 2006). Ants appear to be particularly suitable as models for Batesian mimics because, besides being especially abundant insects in most terrestrial habitats (Hölldobler & Wilson 1990), they are notorious for their defensive adaptations, including powerful mandibles, poison-injecting stings, and ability as social insects to mount communal attacks (Eisner 1970; Blum 1981). Myrmarachne melanotarsa is normally found in the close company of its model, Crematogaster sp. (hereafter Crematogaster), and it is an accurate mimic (see Edmunds 2000) of this ant (Fig. 2). Batesian mimics that closely resemble a particular model species might be expected to live near their model, so as to increase the efficacy of the mimic’s protection from predators that are averse to the model (Edmunds 2000). However, it is also typical to find this small (adult body length c. 3 mm) East African salticid in the company of numerous other conspecific individuals. As a social salticid that mimics a social insect, M. melanotarsa adds a previously unappreciated perspective to our understanding of Myrmarachne-ant relationships. Crematogaster is an aggressive arboreal ant. A common food source of ants, including Crematogaster (Carroll & Janzen 1973), is honeydew, the sugary waste of scale insects (coccids) and other sap-feeding homopterans (Buckley 1987; Völkl et al. 1999). This specialised diet may explain why Crematogaster colonies are common in African and Asian arboreal habitats (Room 1971; Carroll & Janzen 1973; Richard et al. 2001). Preying on the homopterans may provide Crematogaster with a source of protein and lipids (Carroll & Janzen 1973),
Jackson et al.—Behaviour of a social ant-like spider
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Fig. 2 Myrmarachne melanotarsa male (M) and two ants (Crematogaster sp.) (C) walking on nest complex.
while the homopterans may gain a sanitation benefit from Crematogaster’s active removal of excess honeydew (Flatt & Weisser 2000) and a protective benefit from Crematogaster actively driving away not only its own predators but also those of the attended homopterans (Carroll & Janzen 1973). The present study is an initial step towards understanding the biology of M. melanotarsa. Relying on observations from the field and the laboratory, we describe the natural history of M. melanotarsa, particularly as it relates to Crematogaster. We are especially interested in silk mouthing, a distinctive behaviour performed by Crematogaster and M. melanotarsa when on nest complexes. Our experimental findings are an initial step toward clarifying the significance of this unusual behaviour.
MATERIALS AND METHODS Observations in the field and the laboratory Seventy-five colonies of M. melanotarsa (three of which were especially large, containing >50 individuals of M. melanotarsa) were observed almost daily over a 4-month period. Observations were made at different times during the day, and each observation period lasted 20–120 min. Additional observations were conducted using colonies that were established in the laboratory in large plastic cages. Pseudicius spp. and Menemerus spp., the salticids that most often share nest complexes with M. melanotarsa in
nature, ants (Crematogaster), and a large variety of potential prey were maintained in the same cages. The term “colony” is used for nest complexes occupied by M. melanotarsa regardless of whether other salticid species were also present. Experimental procedure In order to determine whether the presence of chemical cues left by ants on silk affected M. melanotarsa’s propensity for “silk mouthing”, we first established, in the laboratory, nest complexes built by M. melanotarsa females. Each nest complex was obtained by putting 20 M. melanotarsa females in a cage at 0900 h and ensuring that prey (chironomid midges) were continuously available for the following 10 days. At the end of the 10-day period, preparation was successful only if all spiders were alive and in a single nest complex. At 0900 h on day 10, all individuals of M. melanotarsa and all prey remains were removed from these testing cages, leaving the intact nest complex within. Each testing cage was assigned at random to one of two groups: (1) Experimental group: 10 Crematogaster workers were placed in the cage immediately after removing the spiders, and all prey remains. Ants were removed from the cages 24 h later (0900 h on day 11); (2) Control group: no ants were put into the cage. There were 35 replicates of each treatment. Five M. melanotarsa females were placed in testing cages immediately following removal of the ants, and then observed for the following 60 min.
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New Zealand Journal of Zoology, 2008, Vol. 35 Fig. 3 Myrmarachne melanotarsa female feeding on juvenile of Menemerus sp.
The outcome of a test was recorded as a “response” whenever at least one of the five spiders mouthed the silk.
RESULTS Location of nest complexes in nature Nest complexes were found on tree trunks, especially in the indentations caused by lost limbs (Fig. 1). Nest complexes were also frequently built around and within the existing silk of either abandoned or still occupied eggsacs of Hersilius caudata, a barkdwelling spider (Hersiliidae) that ambushes prey and builds silken eggsacs on tree trunks (Filmer 1991; Metwally 2001). Observations of silk mouthing Silk mouthing was the dominant behaviour of M. melanotarsa during field observations, and consisted of inserting fangs into the silk, sometimes accompanied by slowly (1–2 times s–1) opening and closing the chelicerae. Immediately before mouthing, the spider usually probed with its forelegs (i.e., its most anterior pair of legs). When probing, it moved these two legs forward and backward so that their
tarsi pushed alternately on the silk (c. 2 cycles s–1; distance moved 0.5–1.0 mm; phasing of the two legs variable, but primarily alternating). However, when mouthing silk, M. melanotarsa stood with forelegs highly flexed, cephalothorax angled 10–45° downward and the front of the cephalothorax (chelicerae and sometimes also the clypeus and anterior medial eyes) pressed against the silk. This posture was usually held for only a few seconds, but there were rare occasions when it was held for more than a minute. The spider finished mouthing by releasing the grip of its fangs on the silk and simply stepping away. However, spiders normally mouthed 10 times or more in one place, then released the silk, stepped about and mouthed again in another location. Experimental results on silk mouthing Myrmarachne melanotarsa mouthed significantly more often in the experimental treatment (ants previously present) than in the control (no ants) (13 of 35 in Experimental, 2 of 35 in Control, Fisher exact test, χ2 = 10.266, P < 0.01). Prey records and predatory behaviour Except for dipterans (midges and mosquitoes), the prey on which M. melanotarsa were found feeding in the field tended to be 1 mm in body length or less,
Jackson et al.—Behaviour of a social ant-like spider and M. melanotarsa in the laboratory appeared to be reluctant to take prey any larger than this. In most instances, when M. melanotarsa was found feeding in the field, the prey was already considerably masticated and could not be identified (Table 1). Salticid eggs and juveniles accounted for the majority of the identifiable prey, followed by hersiliid eggs and juveniles. The remaining prey were insects, especially dipterans (Table 1). Finding M. melanotarsa in the act of feeding on spider eggs and juveniles in the field was difficult, because when feeding on a hersiliid egg or juvenile or on a salticid egg or juvenile, M. melanotarsa was under the hersiliid eggsac silk or the nest-complex silk. Laboratory observations confirmed that M. melanotarsa readily ate not only representatives of all prey categories recorded from the field, including the eggs and juveniles of hersiliids and of heterospecific salticids (Fig. 3), but also aphids, psyllids, whiteflies, mealy bugs, and other unidentified small, soft-bodied insects. Predatory sequences with insect prey, and with juvenile salticids that were out of the eggsac, began when M. melanotarsa oriented from several body lengths away and approached. When close, M. melanotarsa lunged (“lunge” is defined by rear legs remaining on the substrate when the spider suddenly moves its body forward) and grabbed hold of the prey. M. melanotarsa never leapt on prey (“leap” is defined by all legs leaving the substrate). M. melanotarsa did not lunge at the eggs and juveniles of spiders encountered inside eggsacs but instead simply took hold of an egg or juvenile with its chelicerae and pulled it out of the eggsac to feed. Sometimes eggs, however, were not lifted out, but fed on while
229 they remained embedded in the silk. M. melanotarsa also preyed on recently hatched juveniles by lunging at them as they left their eggsacs. M. melanotarsa was never seen attacking adult ants, nor was it ever seen attempting to feed on the ants’ eggs, larvae or pupae as long as adult ants were present. However, by simply grabbing hold with its chelicerae, without first lunging, M. melanotarsa readily fed on unguarded eggs, larvae, and pupae of Crematogaster in the laboratory. When found in the field, M. melanotarsa was usually on the silk of a nest complex (Fig. 2) or at least close by. However, M. melanotarsa sometimes commuted alongside columns of Crematogaster heading to and fro, and these M. melanotarsa individuals were often found 1 m or more from the nearest nest complex. While travelling alongside ant columns, M. melanotarsa appeared to react continually to the nearby ants, actively avoiding contact. There were also instances in which Crematogaster, as well as coccids or other homopterans that feed on sap, were on leaves with individuals of M. melanotarsa. These leaves could be several metres from the nearest nest complex. While on these leaves, M. melanotarsa occasionally fed on honeydew from the homopterans (Fig. 4). Interactions between Myrmarachne melanotarsa and Crematogaster When at a nest complex, M. melanotarsa routinely oriented toward, and briefly displayed at, conspecific individuals that came close, with males tending to display more persistently than females. Displays included specialised posturing similar to that described for other Myrmarachne species (Nelson & Jackson
Table 1 Summary of prey types from observations of Myrmarachne melanotarsa feeding in the field.
Prey type
N
Proportion of all prey (N = 113)
Proportion of identifiable prey (N = 48)
Unidentified Salticid juvenile* Salticid egg† Hersiliid egg Hersiliid juvenile Diptera‡ Psocid Caterpillar
65 16 13 6 2 7 3 1
0.575 0.142 0.115 0.053 0.018 0.062 0.027 0.009
– 0.333 0.271 0.125 0.042 0.146 0.063 0.021
4 Menemerus, 3 Pseudicius, 9 unidentifiable salticids. 7 Pseudicius, 5 Menemerus, 1 conspecific. ‡ 5 Chironomidae (midge), 2 Culicidae (mosquito). * †
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New Zealand Journal of Zoology, 2008, Vol. 35 Fig. 4 Female Myrmarachne melanotarsa on leaf with honeydewproducing coccids.
2007). Generally, the conspecific displayed back briefly, and then the two spiders moved apart. When Crematogaster came near, M. melanotarsa usually oriented without displaying. If the ant approached, M. melanotarsa usually moved away, but sometimes M. melanotarsa males displayed for a few seconds in much the same way as toward conspecific males, then backed away, avoiding contact with the ant. Despite M. melanotarsa’s apparent efforts to avoid physical contact with Crematogaster, there were frequent face-to-face encounters during which M. melanotarsa adopted behaviour strikingly similar to how Crematogaster reacted to conspecific ants. M. melanotarsa, like Crematogaster, cocked its abdomen up almost perpendicular to the substrate and then the spider and the ant “antennated” each other, the ant with its real antennae and the spider with its forelegs. These interactions lasted no longer than a few seconds, and usually ended with both individuals departing by moving past each other. Occasionally, when the ant appeared especially agitated and aggressive, with its abdomen pointing almost forward, M. melanotarsa turned and moved rapidly away. M. melanotarsa never adopted an abdomenforward posture comparable to that of the ant. It was routine to see Crematogaster on the surface of M. melanotarsa nest complexes (Fig. 2) and, as a rule, larger nest complexes harboured more Crematogaster. Sometimes other small (unidentified) ants were present as well, but they were almost always a minority when M. melanotarsa was present. However, there were a few instances in which ants
other than Crematogaster seemed to take over in nest complexes that had formerly been sites of Crematogaster activity, and subsequently the numbers of M. melanotarsa declined rapidly. Large colonies never persisted in the field for long in the absence of Crematogaster. Individuals of Crematogaster seen in nest complexes were usually walking over the silk, frequently stopping and pressing their faces into the silk while opening and closing their mandibles. Sometimes Crematogaster removed prey remains, shed salticid and hersiliid exoskeletons and other detritus that they encountered in the nest-complex silk, and took these items to its own nest. We also observed eggs and recently hatched juveniles of M. melanotarsa and hersiliids being carried in Crematogaster’s mandibles. In these instances, the silk around the nests containing the eggs or juveniles had been torn loose, but ants were never seen tearing the silk and the origin of these tears remains uncertain. However, we confirmed that ants responded opportunistically to silk being torn by using forceps to pull nest silk away (N = 10). In each instance, before tearing the silk, ants were moving in a column adjacent to the nest complex, but the column quickly changed direction and swarmed into the damaged nest complex. M. melanotarsa and other salticid species from the damaged and neighbouring parts of the complex stepped aside and remained nearby, all the while keeping away from the ants which carried away the eggs and recently hatched juveniles exposed by the artificial tears in the nest silk.
Jackson et al.—Behaviour of a social ant-like spider Interactions with other salticids in nest complexes The way other salticids reacted to M. melanotarsa resembled how they reacted to Crematogaster. They oriented towards the ants or the ant mimic and, if they had been walking, they stopped. If the ant or the mimic moved away, the salticid usually watched it and, once the ant or the mimic had moved several body lengths away, the salticid usually walked away in the opposite direction. If approached by an ant, the salticid usually turned and moved quickly away (“ran”) or else first backed away a few millimetres before turning and running. Crematogaster and M. melanotarsa usually showed little response to the other salticids present in the nest complexes, the primary exceptions being instances of close proximity or physical contact. Yet, even when approached by another salticid, M. melanotarsa typically showed no reaction until the other salticid came to within a few body lengths, at which point M. melanotarsa turned and ran, typically without turning to reorient toward the salticid.
DISCUSSION Adaptive advantage of proximity to ants Myrmarachne melanotarsa lives in close proximity to its model, Crematogaster, and this is more unusual than it might first appear. The general pattern for species in the genus Myrmarachne is to live in the same habitat as the ants they mimic, but not usually side by side with the ants (Edmunds 1978; Jackson & Willey 1994). Species that mimic weaverants (Collart 1941; Mathew 1954; Wanless 1978; Nelson & Jackson 2008) are the most comparable to M. melanotarsa, but they do not associate with their model ant species as closely as we have found with M. melanotarsa. Our research suggests that particularly intricate links have evolved between M. melanotarsa and Crematogaster. Our objective in ongoing research on M. melanotarsa is to clarify the cost-benefit tradeoffs that might apply to living in groups and associating with Crematogaster. Here we provide only an overview of this work, and its relationship with the wider literature on spider sociality. Crematogaster generally moves from place to place in columns and sometimes M. melanotarsa joined the marching ants. The Crematogaster columns often led to honeydew-producing coccids, and M. melanotarsa, along with Crematogaster, fed on
231 honeydew (Fig. 4). Reports of salticids feeding on honeydew are scarce. In fact, other than the present study, we know of only one other report (Collart 1929). However, there might be an important lesson to be found by considering the literature on nectar feeding by salticids and other spiders. Until recently, reports of this were rare in the literature, but recent work suggests that it is probably common for salticids to feed from flowers and extra-floral nectaries (Ruhren & Handel 1999; Jackson et al. 2001). Records of nectar feeding might provide a hint that this mode of feeding is particularly prevalent in the genus Myrmarachne. Four of the 31 (12.9%) salticid species seen feeding on nectar in the field (Jackson et al. 2001) were from the genus Myrmarachne. Yet, out of about 5000 described salticid species, only about 200 (4%) are from the genus Myrmarachne (Platnick 2007; Proszynski 2007). It is interesting that the only other report besides ours of a salticid feeding on honeydew came from M. foenisex, another species from the genus Myrmarachne (Collart 1929). Perhaps feeding on honeydew is disproportionately common in salticids that mimic ants, as honeydew-producing insects tend to attract ants and ants tend to deter most other salticids (Nelson & Jackson 2006). The silk-mouthing behaviour of M. melanotarsa and Crematogaster may also be a method by which the ant and the ant mimic can feed on honeydew together. Homopterans make honeydew by excreting only partially digested sap, and possibly ants in turn fail to digest all of the sugar in honeydew before excreting it on nest silk. If so, silk mouthing by M. melanotarsa and the ants may be a way of harvesting sugar excreted by the ant as secondary honeydew. Our observations suggest that, for M. melanotarsa, the risk of ant predation on eggs is an important cost of associating with Crematogaster. Something similar seems to apply to embiopterans, web-spinning insects that resemble social salticids by using silk to spin communal oviposition and resting shelters (Edgerly et al. 2006). Ants are common in the same environments as embiopterans, often making trails directly across the silk (Edgerly 1988). As long as the embiopterans’ silk walls remain intact, the ants remain outside. However, should the silk covering be breached, ants swarm over the embiopteran colony, entering through the breach and attacking the inhabitants (Edgerly 1988). This behaviour is similar to the way Crematogaster reacts to nest complexes of M. melanotarsa being damaged. When we tore nestcomplex silk, Crematogaster swarmed over and into
232 the normally inaccessible interior and, once inside, the ants foraged on eggs and juveniles. Waste and prey remains inevitably build up with so many salticids living close together in nest complexes, which in turn may encourage deleterious accumulation of parasites and pathogens. For social web-building spiders and other animals that live in large, fixed colonies, waste disposal may be a serious problem, and social web-building spiders are known to devote considerable time to cleaning their communal webs (Ebert 1998). For M. melanotarsa, help may be provided by the ants, as Crematogaster workers routinely collected and disposed of prey remains, dead spiders, and other detritus that accumulates in the colony silk. The weighting of these costs and benefits at any one time will determine how we view the relationships between the different participants. Any simple characterisation of how each species is adapted to the other appears unrealistic. However, the conventional Batesian mimicry hypothesis is clearly relevant for understanding the benefits for M. melanotarsa of close physical proximity to Crematogaster because living in the vicinity of ants might lessen predation by predators that have learned or already have an innate aversion to the model (Nelson et al. 2006). Associating with other spiders Myrmarachne melanotarsa is distinctive not only for living in close proximity to ants but also for sharing nest complexes with other salticids, especially Menemerus spp. and Pseudicius spp., and for building nest complexes on and within the eggsacs of Hersilius caudata. In an interesting review, Whitehouse & Lubin (2005) considered the advantages of living in a social group that apply to the individual spider. They argued that, depending on the spider species, these advantages were related to one or more of the following: reproduction, foraging or protection. For M. melanotarsa, it is somewhat more complicated because this social salticid has neighbours of the same and of different species, including Crematogaster, an ant, Hersilius, a non-salticid spider, and other social salticids (from other genera Menemerus and Pseudicius). For M. melanotarsa, foraging advantages are likely, as this ant-mimicking salticid feeds on the juveniles and eggs of the hersiliid and salticid spiders with which it associates. This is an example of brood parasitism, which has been documented in other salticids (Boulton & Polis 2002), including other species of Myrmarachne (Jackson & Willey 1994). M. melanotarsa puts a different twist on brood parasitism because, as a social
New Zealand Journal of Zoology, 2008, Vol. 35 salticid that mimics a social insect (Crematogaster) that also preys on eggs and juveniles opportunistically, groups of M. melanotarsa might be especially effective at gaining access to eggs and juveniles. We are currently investigating a particular hypothesis: that groups of M. melanotarsa, by resembling groups of Crematogaster, intimidate other salticids and thereby gain access to the other salticids’ eggs and juveniles. At first sight, it appears maladaptive for Menemerus spp. and Pseudicius spp. to share nest complexes with M. melanotarsa. However, there may be compensating advantages, and these fall, for Menemerus and Pseudicius, within the protective function of sociality. As many ants are predatory, territorial and highly aggressive (Hölldobler & Wilson 1990), routinely attacking potential predators of salticids that come close, the immediate vicinity of ants and ant mimics could be a safe haven for a salticid. For these salticids, ants and Myrmarachne would have a role similar to the role of protector species in mixed-species bird flocks (Pius & Leburg 1998; Richardson & Bolen 1999), where one bird species (the “protector”) is more effective at driving predators away and other species in the same flock benefit by associating with the protector species. For the salticids, associating with ants and with M. melanotarsa incurs costs (loss of eggs and small juveniles), but these may be compensated for by the protector-species benefits. Collective mimicry Batesian mimicry in the genus Myrmarachne is typically envisaged as individual ant mimics benefiting from their resemblance to individual ants (Cushing 1997). However, by living in groups, M. melanotarsa has taken ant mimicry a step further. Ants are social insects, and potential predators often encounter groups of ants rather than encountering ants one at a time. Crematogaster, for example, whether at a nest or travelling along a trail, is seldom alone and, if one ant is attacked, a swarm of conspecific individuals in the vicinity normally come to its defence and ward off the predator. Simply witnessing a large group of ants may be enough to deter many predators. This argument suggests a hypothesis we are currently investigating (the “collective mimicry hypothesis”). We propose that, for some of its predators, a group of M. melanotarsa spiders resembles a swarm of Crematogaster. The collective mimicry hypothesis might account for M. melanotarsa’s tendency to maintain especially close physical proximity to its model.
Jackson et al.—Behaviour of a social ant-like spider For people, and presumably for some of M. melanotarsa’s natural predators (see Nelson & Jackson 2006), a mixed-group consisting of M. melanotarsa and Crematogaster may be hard to distinguish from a pure group of Crematogaster, suggesting that, for an individual of M. melanotarsa, the prime objective is to be close to other similar individuals. Whether the other individuals are conspecifics or the model may be relatively unimportant. Collective mimicry appears to be an example of sociality having a role in predator protection as characterised by Whitehouse & Lubin (2005): “the sum of individual behaviours in the group that enhance the survival of the group members…in the presence of predators or parasites”. However, M. melanotarsa seems to differ from the examples reviewed by Whitehouse & Lubin (2005) because, in the case of M. melanotarsa, an anti-predator advantage of sociality seems to have an unusual link to Batesian mimicry. Here we have an example of an ideal convergence between the independent protective functions of social living and of Batesian mimicry of a social model.
ACKNOWLEDGMENTS We thank the staff at ICIPE for numerous ways in which they supported the research. For technical assistance at ICIPE, we thank Godfrey Sune, Stephen Alluoch, Silas Ouko Orima and Jane Atieno. This research was assisted by grants to RRJ from the Royal Society of New Zealand (Marsden Fund and James Cook Fellowship) and the National Geographic Society.
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